Steam cracking, chemistry

Gas-phase, steam cracking reactions dominate the chemistry of biomass gasification. At temperatures above 650°C, these reactions proceed very rapidly and generate a hydrocarbon rich syngas containing commercially interesting amounts of ethylene, propylene, and methane. Increased pressure appears to inhibit the gasification process. 

The first question to be answered is related to the type of models actually used for gas phase reactions. Similar evolutions took and take place for the models used for steamcracking, atmospheric and stratospheric chemistry, engines, etc. Very typical is the situation related to the pyrolysis reactions and more particularly the steam-cracking of hydrocarbons. 

Dancuart, L.P., Mayer, J.F., TaHman, M.J., Adams, J., 2003. Performance of the Sasol SPD naphtha as steam cracking feedstock. Preprints — American Chemical Society, Division of Petroleum Chemistry 48, 132. 

Low-pressure steam-generating Cl models are particularly sensitive to changes in water chemistry, and their rough waterside surfaces makes them susceptible to deposition, and thus to the risk of overheating and subsequent cracking. Typically, they are quite small boilers, most being less than 200 hp. 

In 1962 Mobil Oil introduced the use of synthetic zeolite X as a hydrocarbon cracking catalyst In 1969 Grace described the first modification chemistry based on steaming zeolite Y to form an ultrastable Y. In 1967-1969 Mobil Oil reported the synthesis of the high silica zeolites beta and ZSM-5. In 1974 Henkel introduced zeolite A in detergents as a replacement for the environmentally suspect phosphates. By 2008 industry-wide approximately 367 0001 of zeolite Y were in use in catalytic cracking [22]. In 1977 Union Carbide introduced zeolites for ion-exchange separations. 

Methanol Formaldehyde Ethylene Propylene oxide Phenol 1,4-Butanediol Tetrahydrofuran Ethylene glycol Adipic acid Isocyanates Styrene Methyl methacrylate Methyl formate Two-step, via CH4 steam reforming Three-step, via methanol Cracking of naphtha Co-product with t-butyl alcohol or styrene Co-product with acetone Reppe acetylene chemistry Multi-step Hydration of ethylene oxide Multi-step Phosgene chemistry Co-product with propylene oxide Two-step, via methacrolein Three-step, via methanol... 

Propylene, or propene by lUPAC (International Union of Pure and Applied Chemistry) nomenclature, is probably the oldest petrochemical feedstock, employed as it was in the early processes to isopropanol. It is produced by the cracking of propane or higher hydrocarbons in the presence of steam... 

The effects of temperature and steam on the deterioration of catalysts employed in cracking processes are under continuous surveillance, and these effects may be studied in a rather straightforward manner by physical property measurements independent of the chemistry of the surface. For example, as will be shown later, if a significant increase in pore radius accompanies loss in area, steam deactivation is probably indicated. Moreover the pore structure as determined by adsorption techniques is undoubtedly related to the ease of admission of reactant molecules and the diffusion out of product molecules as well as to the regeneration properties when carbonaceous deposits must be removed. 

Union Carbide pioneered the development of products derived from ethylene and propylene in the 1920 s and 30 s. Our original cracking furnaces were designed and built with very limited knowledge of the chemistry of hydrocarbon pyrolysis. Information, especially quantitative information, on the effect of such variables as pressure, partial pressure of furnace gas, and the time/temperature relationship was almost nonexistent. Steam was added to the cracking mixture early - but only because it was found that steam eliminated formation of carbon particles which were rapidly eroding out furnace tube bends. 

Special attention should also be paid to the selection of materials and to the coolant chemistry, which also make an important contribution to the reliability of the steam supply system for the nuclear plant. The compatibility of materials and coolant, which is of the utmost importance to minimizing the amount of maintenance, repair and statutory inspection necessary for primary circuit components, should be given careful consideration. Only those materials should be used that have been shown to be compatible with the coolant under the conditions (of temperature of coolant and material and coolant composition) that will prevail in the reactor. A specific concern is the possible occurrence of intergranular stress corrosion cracking. 

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